1. Field of the Invention
The present invention relates to near-field optical microscopy in which a sample undergoing study is irradiated with light and the reflection/transmission or absorption characteristics of the sample are analyzed so that an image of the sample is generated. The invention also relates to near-field optical (NFO) microscopes in which a near-field optical probe is used to produce changes in the reflection, transmission or absorption characteristics of a sample under study.
2. Discussion of the Related Art
Optical microscopy is one of the most widespread experimental techniques in science because of its ease of use and its well established theoretical background, which makes image interpretation relatively straight forward. The major limitation of optical microscopy is on the resolution of the produced images, which will generally be equal to the order of wavelength of light which is used. This limitation remains despite the fact that under certain circumstances distances or thicknesses can be measured with much higher accuracy. In these cases, the sample geometry is restricted or certain predetermined information about the sample is necessary in order to interpret the images. Confocal microscopy and video enhanced microscopy can increase lateral and vertical resolution by typically a factor of two for some types of samples (see Pluta, M. Advanced Light Microscopy; Elesevier: Amsterdam, 1989; volume 2).
An improvement in resolution is possible by using a near-field optical microscope. The basic idea for this type of microscope originated in Ireland in the beginning of the 20th Century and was demonstrated using microwaves in the 1970's. The first application to optical light was reported recently in Pohl, D. W.; Denk, W.; Lanz, M. Applied Physics Letters, 1984, volume 44, pages 651-653. In near-field optical microscopy, a sharp tip which is usually a sharpened glass fiber, is used as a light source and raster scanned above the surface. Using the specially sharp fibers, a resolution of about 12 nanometers can be achieved. The sharpening of the fibers is usually done by pulling them in a very special way which results in a very thin fiber. They are very fragile, yet need to be scanned very close to the samples. Care needs to be taken to prevent the fiber from touching the sample and breaking. One solution to this problem involves looking at the lateral bending of the fiber and using this signal as the input to a distance control circuit. Another problem with using fibers is their low efficiency. Because of their small opening angle, most of the light is absorbed by the metal coated fiber walls so that only a small fraction of the light passes through the tip of the fiber (signal levels of up to 50 nW have been reported). Despite these problems, the benefit of the increased resolution has justified the increased amount of work in this area and has yielded impressive images. Another approach to the low resolution problem has been to make specially designed microfabricated tips for scanning near-field optical microscopy similar to those designed for scanning ion conductance microscopy. These tips are not, however, available commercially and the microfabrication process is too complex to make it practical for most researchers.
Near-field scanning microscopy has been performed in the prior art mainly using optical fibers which have been tapered to a sharp tip so as to project light onto the sample in order to measure light reflected by the sample and/or transmitted through the sample and projected onto a detector device for viewing and/or analysis by an operator. However, the use of optical fibers for use in near-field optical microscopy has other disadvantages in that these optical fibers are subject to easy breakage, they can not be fabricated with current micro-machining technology and the fibers can be destructive to the sample if they come in contact with the sample, such as, by scraping the sample. Therefore, a complicated mechanism for preventing the contact between the optical fibers and the sample has been necessary. The use of optical fibers has another drawback in that it is difficult to taper the optical fibers in order to create an aperture small enough for performing high resolution imaging.
In order to overcome such disadvantages associated with optical fibers, there has been proposed a method of performing near-field optical microscopy using a cantilever having a sharp tip formed of silicon nitride (see van Hulst, et al, "Operation Of a Scanning Near Field Optical Microscope in Reflection in Combination With a Scanning Force Microscope", SPIE Vol. 1639, pp. 36-43, 1992). In this conventional device as shown in FIG. 1, a micro-fabricated SiN cantilever 131 having an integrated pyramidal tip 123 is fixed at an angle to the upper surface of a sample 121. The tip is coated with an opaque material so that only light focused on the aperture of the tip passes through to the sample surface. The sample is placed on a mount 127 which is movable with respect to the fixed cantilever via a piezoelectric tube scanner 119. As can be seen from FIG. 1, light from a source 143, such as a mercury discharge lamp, passes through lenses 141 and 139, and is reflected by a dichroic mirror 137 and focused by objective lens 111 onto the back surface of the sample to illuminate the sample portion under study so that it can be viewed by an observer. Light from a laser source 125 is projected through a dichroic mirror 135 and is deflected by reflector 129 and focused by lens 133 into the back portion of the silicon nitride tip of the cantilever 131. Light which passes through the opening of the tip 123 is reflected off the sample back through the silicon nitride tip, reflected by elements 129 and 135, passed through a filter 113 and diaphragm 115 and is then received by a detector 117. These detected reflections are then used to generate an image of the sample which has been scanned across its entire surface.
However, the foregoing apparatus has an important disadvantage in that the light which is projected onto the back surface of the cantilever tip is also reflected back to the detector device along with the light which passes through the tip of the cantilever, strikes the surface of the sample and is then reflected back through the tip to the detector device. Because of this, it is difficult to distinguish the small changes due to the light which is reflected off the sample since there is a significant amount of reflected light which constitutes "noise" generated by the reflections off the back surface of the cantilever tip.
Another known method of performing near-field optical microscopy relies upon local modification of an evanescent field which is an extremely thin (of micron proportions) region of electromagnetic energy which exists adjacent to a back surface of a prism where a beam of light has been projected through a front surface of the prism and this light beam is totally internally reflected off the back surface of the prism so that no visible light is emitted through the back surface of the prism into the far field. The introduction of a sharp probe tip into this evanescent field, however, will cause an induction of some of the electromagnetic energy to be emitted from the back surface of the prism. This will result in the emission of light into the far field. This light which is emitted from the back surface of the prism will be modified by the different properties of the sample which is disposed on the back surface of the prism where the back surface serves as a mount for the sample and the probe tip follows the sample topography. An image of the sample can be obtained by detecting the changes in light emitted through the sample into the far field as the probe tip follows the surface topography of the sample. However, the main disadvantage of using evanescent field modification is, as noted above, that these fields extend only a very small distance above the back surface of the prism. Therefore, if it is desired to view an object such as a cell, for example, which is of relatively thick dimensions, the foregoing prior art devices will not provide satisfactory results, since the evanescent field would not extend completely through the relatively thick cell under study so as to be able to be locally modified by the probe tip.
Another known method of performing near-field optical microscopy using a probe is disclosed in U.S. Pat. No. 5,105,305 to Betzig et al. According to this method and apparatus, a microscope is provided with a probe having a narrow aperture which includes a plurality of minute particles which are impregnated with fluorescent dye so that upon irradiation of these particles with light, the particles emit fluorescent light to be projected onto the sample. Then, by moving the probe aperture into close proximity to the sample surface, the fluorescence emitted from the probe aperture will be quenched due to dipole-dipole interactions between the particles and the sample surface. The Betzig et al method and apparatus also provides transmission and reflection modes of scanning using the fluorescent light emitted from the probe tip in order to generate images of the sample surface or composition of the sample. Such a microscope has a main disadvantage, however, that the probe apparatus is not finely controlled with respect to its positioning in close proximity to the sample surface which could thereby lead to the possibility of a large contact force with the sample surface which may damage the sample.
Another known probe for performing near-field optical microscopy is disclosed by Pohl in U.S. Pat. No. 4,604,520. Such a probe is formed by providing a transparent tip having a metal coating around the outside walls thereof in order to provide a small opening for performing the optical scanning in a transmission or reflection mode. However, the Pohl apparatus has the same disadvantages described above with respect to Betzig et al, i.e., there will be the possibility of causing damage to the sample if the probe tip comes too close to the sample surface.